Apoptosis is the term used to describe a type of cellular death that occurs in many tissues as a normal physiological process. Also called "programmed cell death," this form of cellular demise involves the activation in cells of a built-in genetic program for cell suicide by which cells essentially autodigest. The remnants of these dead cells are then cleared almost without a trace by neighboring phagocytic cells, without resulting in inflammation or scarring. Apoptosis thus stands in marked contrast to cell death caused, for example, by oxygen-deprivation in the settings of myocardial infarction or stroke, where cells lose their energy supplies, rupture and spill their contents into the extracellular milieu.
In addition to the normal physiological process where cells are turned over within the body, apoptosis can be induced to occur by cellular, hormonal or other stimuli to remove unwanted cells from the body. For example, killing of tumor cells and virus-infected cells by the immune system's cytolytic T-cells occurs via apoptosis following target recognition. Apoptosis also occurs via loss of hormonal stimulation in the female reproductive tissues with each menstrual cycle in the absence of a successful pregnancy. Further, numerous studies have shown that apoptosis accounts for cell death in a wide variety of clinically important areas. For example, essentially all chemotherapeutic drugs currently used in the treatment of cancer, as well as x-irradiation in many cases, ultimately kill malignant cells by activating intracellular pathways leading to apoptosis.
In contrast to the effect of apoptosis in normal cellular phenomenon, when aberrantly regulated, the death of cells through apoptosis can lead to a variety of disease states and pathological conditions. For example, the death of neurons that occurs in diseases such as Alzheimer's dementia and Parkinson's disease shows many hallmarks of apoptosis. Autoimmune diseases, where immune cells inappropriately attack normal tissues, is due, in part, to a failure of apoptosis to occur. Additionally, cell death caused by viral infection can occur through apoptosis in many cases, including T-cell death induced by the Human Immunodeficiency Virus (HIV) that causes AIDS. In contrast to the induction of apoptosis caused by some viruses, other viruses inhibit this process through the expression of gene products that block apoptosis. Herpes Simplex virus is a specific example of this inhibition where the prevention of apoptosis is necessary for its characteristic persistent or "latent" viral infection.
Efforts have been made using conventional chemotherapy to treat many of the disease states that result in inappropriate apoptotic cell death, including all of those mentioned above, but have so far yielded only minor progress toward an effective treatment. Additionally, other non-conventional approaches have also been tried in specific enhances. For example, Parkinson's disease has been treated in humans using fetal neural tissue transplantation. Extensive testing of such neural tissue transplants as well as testing of genetically engineered fibroblasts has been investigated in animal models of Parkinson's and Alzheimer's disease. In the latter case, fibroblasts were modified to secrete neurotrophic factors such as nerve growth factor(NGF) or neurotransmitters such as precursors of dopamine to prolong recipient neural cell survival. The general difficulty of these treatments is that either only a small number of the transplanted cells initially survive upon implantation, or in the case of genetically-engineered fibroblasts, many of the cells that initially survive fail to continue long-term in vivo survival.
As mentioned previously, cancer chemotherapy acts through a variety of intracellular targets which culminate in the activation of the apoptotic pathway. Cancer is the second leading cause of death in the United States. One out of every 3 Americans will develop some form of this disease in his or her lifetime, and the vast majority will die as a direct result of their malignancies, primarily because of the inadequacy of currently available chemotherapeutic drugs or the spontaneous resistance of malignant cells to such treatments.
Although many details of malignancies are not fully understood, the basis of a variety of cancers have been worked out in large part. For example, the unregulated expression of many genes is now known to cause oncogenic transformation. These genes have generically been termed oncogenes because of their transformation ability when abnormally expressed. Typical examples of such oncogenes include protein kinases such as Src, Herb-2 (NEU), and BCR/ABL, GTPases such as Ki-RAS, Ha-RAS, and N-RAS, and transcription factors such as Fos, Jun and Myc. These and other oncogenes have been well documented to be a major cause in the malignant phenotype of cancers such as neuroblastoma (N-myc), Burkitt lymphoma (c-myc), colorectal carcinoma (Ki-RAS), chronic myelogenous leukemia (BCR/ABL), breast cancer (Herb-2/NEU), and lung cancer (L-myc). Attempts have been made to target several of these oncogenes to provide a therapeutic treatment for the relevant cancer. However, because of the problems mentioned above in regard to chemotherapy, such attempts all too often have proven to be of marginal benefit.
In contrast to oncogenes whose abnormal or unregulated expression results in increased proliferation, there is at least one "oncogene" whose overexpression results in prolonged cell survival. This oncogene is termed Bcl-2 and was originally discovered because of its inappropriate activation in lymphomas, cancers of the lymph nodes, where it contributes to neoplastic cell expansion. High levels of Bcl-2 protein have been demonstrated to occur in approximately 50,000 new cases of lymphoma and leukemia each year in the United States alone, essentially all cases of drug-resistant prostate cancer (150,000 cases per year in USA), 80% of nasopharyngeal carcinomas, about 70% of breast cancers (approximately 100,000 cases per year in the United States) and all cases of colorectal carcinoma examined to date (110,000 new cases per year in USA).
Since its initial discovery, it has been shown that Bcl-2 is also normally expressed in many tissues of the body, where it serves a physiological role of maintaining the survival of long-lived cells. Among the types of cells whose survival Bcl-2 regulates are the "memory" lymphocytes that are generated during immunizations, many types of neurons in the brain and particularly in the peripheral nerves that control muscle and organ functions, bone marrow cells, skin, and the stem cells giving rise to the absorptive cells that line the gastrointestinal tract.
From the foregoing discussion, it is apparent that for many of the diseases that impact the human species, there have been no major advances toward an effective treatment within the last 10 to 15 years. However, as diverse as these diseases and their treatments are, there has developed one common mechanism by which they manifest their characteristics or by which chemotherapy has been rendered ineffectual. This common mechanism is either the induction or inhibition of programmed cell death.
Thus, there exists a need to control the process of apoptosis in order to generically treat a broad range of diseases and pathological conditions, and also to be able to augment clinical treatments employing readily available drugs. The present invention satisfies this need and provides related advantages as well.